There are containers that are, after being filled with a food product and sealed, put into a retort chamber for thermal sterilization at a temperature above 100.degree. C. (for about 20 minutes at 127.degree. C. for example). As one of such containers, a collapsible tube container comprising a container body and a cap, both made of the thermoplastic materials, has been known (see Japanese Patent Kokai (A) Publication No. 56-106753 (1981)).
The aforementioned collapsible tube container has an aperture at the top end of a neck, which is sealed with a thin film including aluminum foil, and an open bottom end. This container is used in such manner that, after a cap is screwed onto the neck, the tube container is placed up side down and filled with a food product in the form of paste through the open bottom end. After the bottom end is pressed flat and heat-sealed, the tube container is put into a retort chamber and is subjected to thermal sterilization.
Such collapsible tube containers each comprising the container body and the cap which are made of thermoplastic materials and engaged with each other in threadable arrangements easily deform as the container body and the cap are softened by heat during the thermal sterilization process in the retort chamber. Also, in the retort chamber, the containers are subjected to fluidal pressure from sprays of hot water and cooling water as well as from the product which is agitated for the purpose of accelerating heat transfer, so that rotating force exerted to the caps about the container bodies. This causes the caps of the majority of the containers to be loosened and in the worst case, to be completely unscrewed off the container bodies.
As the aperture at the top end of the necks of the containers, is sealed with a thin film such as a heat-sealing propylene tape which includes a gas barrier layer of aluminum foil, the product may not leak out of the containers even when the caps are loosened or completely unscrewed.
However, if consumers, who are going to buy a food product in such containers, find that the caps are loosened, they may suspect that the containers have been unsealed, and lose confidence in the product. Thus loose caps are undesirable for a supplier of the products, and the supplier is often forced to retighten the caps or reset the caps which have come off the containers.
Therefore, a retortable container having a cap which may not become loose during retorting process has long been desired in the industry.
As a container employing an anti-loosening mechanism for preventing threadable engagement of its container body and its cap from loosening, there has been known a container comprising a container body 31 made of glass and having a threaded neck 32 and a screw cap 35 made of a synthetic resin for threadable engagement with the neck 32 as shown in FIG. 4 (see Japanese Utility Model Kokai (A) Publication No. 55-117952 (1980)). With this container, the neck 32 has a first engagement ridge 34 extending around the periphery at the top end in an outer diameter smaller than the inner diameter of female threads 39 of the cap, and the cap 35 has a second engagement ridge 40 provided at the upper end of the inner surface of a circumferential wall 38 in an inner diameter layer than the outer diameter of ridge 34, whereas screwing the cap 35 with sufficient force onto the neck 32 causes the second engagement ridge 40 to pass over the first engagement ridge 34 thereby engaging onto the lower surface thereof, and unscrewing the cap 35 with sufficient force causes the second engagement ridge 40 to pass over the first engagement ridge 34 upwardly.
Such conventional anti-loosening mechanism require that the second engagement ridge 40 deforms upward and at the same time the circumferential wall 38 of the cap 35 deforms outwardly, so that the second engagement ridge 40 on the cap may pass over downwardly the first engagement ridge 34 on the neck.
However, the circumferential wall 38, which is the threadable engagement portion of the cap 35, is usually formed to have a relatively thick wall in order to prevent deformation when screwed, and is consequently difficult to deform outwardly. Particularly, portions of the circumferential wall close to a top wall 37, where the second engagement ridge 40 is located, may not deform outwardly unless substantial force is exerted.
For this reason, an intensive rotating force is required to have the second engagement ridge 40 pass over the first engagement ridge 34 and, as a result, the rotating force exerted on the female threads 39 of the cap 35 during the screwing and unscrewing essentially accelerates wearing of the threads.
Further, when upper portions of the circumferential wall of the cap having the second engagement ridge 40 deform outwardly, the threaded portions of the circumferential wall below ridge 40 tend to deform greater than the second engagement ridge portions, so that repetitive screwing and unscrewing of the cap 35 causes permanent deformation of the threaded portions of the cap, resulting in undesirable looseness of the threadable engagement.
An object of the present invention is to provide a retortable container having a screw cap which may not be loosened or unscrewed completely off the container body during a retorting process.
Another object of the present invention is to provide a retortable container having a screw cap which can be mounted to and detached from the container body with a relatively small force.
A further object of the present invention is to provide a retortable container having a screw cap which allows, through openings, for reading draining water that has entered gap portions between the neck and the cap during sterilization, and for quickly getting the gap portions dry with drafts through the opening.